OENFRAL CHARACTERS OF REPTILES. 541 



as upon the oxygen in a gaseous state. In some Reptiles, the 

 cutaneous respiration is even so active, that in certain circum- 

 stances it is sufficient for the maintenance of life. The lungs 

 are organised in a manner which is not favourable to a great 

 activity of their functions ; for their air-cells are very large, and 

 consequently the vascular surface destined to receive the contact 

 of the air is of small extent (ANiM. PHYSIOL., 312). They 

 are not lodged in a distinct cavity ; the thorax not being separated 

 from the abdomen by a diaphragm ; and the air is renewed in 

 their interior with less facility and regularity than in the higher 

 animals. Sometimes even, as in Frogs, and Tortoises, the 

 absence of the ribs, or the immobility of these bones, renders 

 the ordinary respiratory movements impossible ; and then it is 

 by an act df deglutition, that the air is forced into the lungs. 

 It is likewise to be noticed, that Serpents possess, in the arrange- 

 ment of the respiratory apparatus, a remarkable anomaly ; one 

 of their lungs being so little developed, that we can often scarcely 

 perceive it ; whilst the other acquires very considerable dimen- 

 sions (Fig. 315). 



480. Reptiles are all cold-blooded animals ; that is to say, 

 they do not produce enough heat, to have a temperature sensibly 

 higher than that of the surrounding air. The whole of their 

 body is heated or cooled at the same time as the surrounding 

 medium; and the changes of temperature which they thus 

 experience, have great influence upon all their functions. A 

 warmth of from 105 to 120 is soon fatal to most of these 

 animals; and cold tends to depress all their vital operations. 

 In winter, most of them could no longer digest the food intro- 

 duced into their stomach, and do not take nourishment. Their 

 respiration also diminishes in a most remarkable manner. Thus 

 during the cold season, the action of the air on the skin is suffi- 

 cient for the maintenance of the life of a Frog ; and the lungs of 

 one of these animals may be removed without producing asphyxia; 

 whilst in summer they have need not only of the pulmonary 

 respiration, but also of their cutaneous ; and death soon occurs 

 when the ah? does not act on the skin, or is excluded from the 

 lungs. Limitation of the supply of air often produces in these 



